HDRLightStudio on 4K screen I am sorry you are experiencing a problem. Supporting HiDPI displays is tricky and I don't think we have it right yet based on what you are experiencing. There are many variables and even bugs in Windows OS on how it's handling HiDPI depending on if you mix resolutions across different panels too.
To help we implemented a windows environment variable to further control the UI scaling in HDR Light Studio, for people with issues like you.
HDRLS_SCALE_GUI
Forces scale of the User Interface through HDRLS_SCALE_GUI environment variable. The value represents the scale size you want. 1 = 100%, 2 = 200% etc.
For example, to double the scale of the UI:
HDRLS_SCALE_GUI = 2
Please try setting this, and then restarting HDR Light Studio, see if this gives you the control you need over the UI, thanks.